A natural gas produced from a wellhead is subjected to liquefaction pretreatment for separating impurities, and then cooled to be liquefied, and shipped as a liquidized natural gas (LNG).
Examples of the impurities to be separated from the natural gas through the liquefaction pretreatment include non-hydrocarbon gases, such as a carbon dioxide gas, a nitrogen gas, and a hydrogen sulfide gas, and as well, mercury and water.
As a non-hydrocarbon gas separation device configured to separate a non-hydrocarbon gas from a natural gas, there is given a separation device utilizing a gas separation membrane. This kind of non-hydrocarbon gas separation device is hardly accompanied by a phase change at the time of gas separation, and is configured to perform gas separation by using, as a drive energy, a difference in pressure (difference in partial pressure) of a gas to be separated before and after its permeation through the gas separation membrane, and utilizing a difference in gas passage speed through the gas separation membrane. Therefore, such method has high energy saving performance and easy handleability as compared to a cryogenic separation method, a pressure swing absorption (PSA) method, and an absorption method involving using an absorption liquid.
For example, in Patent Literature 1, there is a description of a carbon dioxide separation system in which a primary carbon dioxide separation device equipped with a zeolite membrane for carbon dioxide separation and a secondary carbon dioxide separation device that employs an amine absorption method or a PSA method are connected in series. The carbon dioxide separation system has an object to remove carbon dioxide from a gas that has undergone an exhaust gas shift reaction in an integrated coal gasification combined cycle plant or from a natural gas, and thus produces a carbon-dioxide-removed gas having a carbon dioxide concentration of 2% or less from a mixed gas having a carbon dioxide concentration of from 3% to 75%.